There are many examples of large arrays of functional components which can provide, produce or detect electromagnetic signals or chemicals or other characteristics. An example of such a large array is that of a display where many pixels or sub-pixels are formed on an array of electronic elements. For example, an active matrix liquid crystal display includes an array of many pixels or sub-pixels which are fabricated on amorphous silicon or polysilicon substrates which are large. As is well known in the art, it is difficult to produce a completely flawless active matrix liquid crystal display (LCD), when the display area is large, such as the LCD's on modern laptop computers. As the display area gets larger and larger, the yield of good displays decreases. This is due to the manner in which these display devices are fabricated.
Silicon VLSI can be used to produce such an array over a silicon wafer's surface, but silicon wafers are limited in size, limited in conductivity, and not transparent. Further, processing of large areas on silicon wafers can be expensive. Displays which valve the light coming through them need to be transparent. Single crystal silicon can be bonded to a glass substrate and then etched to remove most of the area to achieve transparency, but this is intrinsically wasteful in that, for the sake of maximizing light transmission, the majority of the processed material is discarded and becomes chemical waste. The under-utilization of the precious die area wastes resources, causes greater amounts of chemical waste to be generated in the process, and is generally inefficient and expensive. Another example is photodiode arrays which may be used to collect solar energy. Large arrays of silicon photodiodes with concentrating lenses have been made by sawing wafers and using a pick and place assembly, but thermal dissipation is poor for large elements, and the small elements require too much assembly time.
Alternative approaches to fabricating arrays such as displays include fabricating the desired circuitry in an amorphous or polycrystalline semiconductor layer which has been deposited on a substrate, such as glass or quartz. These approaches avoid the limitations of the size of the available single crystal silicon wafers, and avoid the cost of the single crystal wafers, but require expensive deposition of the semiconductor layer, and they still require processing of the entire large substrate to form the active elements in an array, still resulting in the production of much chemical waste and wasted resources. These processes also limit the choice of the substrate; for example, plastic substrates cannot be used due to the nature of the processes which deposit the semiconductor layers. Furthermore, amorphous or polycrystalline silicon semiconductor elements do not perform as well as those made from single crystal semiconductor material. For displays, as an example, it is often difficult or impossible to form some of the desired circuitry out of the amorphous or polycrystalline semiconductor materials. Thus, high frequency edge drivers may be impossible to form out of these materials. This results in the difficulty and expense of attaching an electrical lead for each and every row and column of an array, such as an active matrix liquid crystal display array.
As noted above, another difficulty with the existing techniques is that the large number of elements in a large array results in a low probability that all of them will work properly and thus the yield of acceptably good arrays from the manufacturing process is low. Furthermore, there is no possibility of testing any of the elements until the assembly is complete, and then any imperfection in the array must be tolerated or the entire array could be discarded or special and expensive techniques must be used to repair it. These problems result from the fact that the various elements in the array are fabricated on the array rather than separately.
It is possible to separately produce elements, such as pixel drivers and then place them where desired on a different and perhaps larger substrate. Prior techniques can be generally divided into two types: deterministic methods or random methods. Deterministic methods, such as pick and place, use a human or robot arm to pick each element and place it into its corresponding location in a different substrate. Pick and place methods place devices generally one at a time, and are generally not applicable to very small or numerous elements such as those needed for large arrays, such as an active matrix liquid crystal display. Random placement techniques are more effective and result in high yields if the elements to be placed have the right shape. U.S. Pat. No. 5,545,291 and U.S. Pat. No. 5,904,545 describe methods which use random placement. In this method, microstructures are assembled onto a different substrate through fluid transport. This is sometimes referred to as fluidic self assembly (FSA). Using this technique, various blocks, each containing a functional component, may be fabricated on one substrate and then separated from that substrate and assembled onto a separate substrate through the fluidic self assembly process. The process involves combining blocks with a fluid and dispensing the fluid and blocks over the surface of a receiving substrate which has receptor regions (e.g. openings). The blocks flow in the fluid over the surface and randomly align onto receptor regions.
Thus the process which uses fluidic self assembly typically requires forming openings in a substrate in order to receive the elements or blocks. Methods are known in the prior art for forming such openings and are described in U.S. Pat. No. 5,545,291. The substrate having openings in the glass layer 10 may be used as a receiving substrate to receive a plurality of elements by using a fluidic self assembly method. FIG. 1A shows an example where a separately fabricated element 16 has properly assembled into the opening 14. However, it has been discovered that at times, an element 16 will not properly assemble into an opening 14 due to the fact that the element 16 becomes turned upside down and then lodges in the top of the opening 14. An example of this situation is shown in FIG. 1B. Often times, the inverted element 16 lodges into the opening 14 so tightly that it remains in the opening and prevents non-inverted elements from falling into the opening 14. Thus, the opening at the end of the assembly process will typically not be filled with an element or perhaps worse, may still contain an inverted element lodged at the top of the opening 14.
FIGS. 2A through 2D show an example in the prior art for creating a plurality of openings in a receiving substrate which is designed to receive a plurality of separately fabricated elements which are deposited into the openings through fluidic self assembly. The method shown in FIGS. 2A through 2D begins by, in one example, thermally growing a silicon dioxide layer on a silicon substrate 20. The resulting structure is shown in FIG. 2A with the silicon dioxide layer disposed over the silicon substrate 20. Then, a photoresist material may be applied, and exposed through a lithographic mask and then developed to produce a patterned mask formed from the developed photoresist. Then an etching solution is applied to etch through the patterned mask to create an opening 24 in the silicon dioxide layer 22. The resulting structure is shown in FIG. 2B. Then, the silicon dioxide layer 22 with its opening 24 is then used as a patterned mask to etch the silicon layer 22 to create the opening 26 in the silicon layer 20 as shown in FIG. 2C. This etching of the silicon layer 20 may be performed with a KOH etchant or with an EDP etchant as described in U.S. Pat. No. 5,545,291. After etching the opening 26 in the silicon layer 20, the silicon dioxide layer 22 is removed, for example, by an etch in a hydrofluoric acid solution. This results in the structure shown in FIG. 2D where the opening 26 is now ready to receive a separately fabricated element through an assembly process, such as for example, fluidic self assembly or perhaps a pick and place procedure. The structure shown in FIG. 2D has the drawback that a monocrystalline silicon layer is required in order to use the KOH etch to form the hole.
Even if the opening is perfectly formed, the fluidic self assembly process can still produce poor results if the elements do not flow smoothly over the surface containing the openings. For example, if there is too much friction between the elements and the surface, the elements tend to stick to the surface and do not flow into receptor regions. Thus, receptor regions are not filled or take longer to fill. If the elements remain stuck to the surface during planarization and interconnect metallization, the completion of an assembly with these stuck elements will potentially destroy metal electrical interconnects or other functional features.
From the foregoing discussion, it can be seen that it is desirable to provide methods for allowing improved fluidic self assembly.